The invention relates to a method for recognizing misfires in a piston-type internal-combustion engine having spark ignition, and an engine timing for actuating electromagnetic actuators to operate the cylinder valves, which respectively have an electrical opening magnet and an electrical closing magnet.
In piston-type internal-combustion engines having spark ignition, so-called SI engines, high requirements are placed on a so-called on-board diagnosis, particularly if the engines are installed in motor vehicles. One such requirement is the recognition of misfires. The conversion of this type of diagnosis was previously based on an analysis of the uniformity of rotation supported by signals picked up from the crankshaft. This signal detection is not precise enough, however, because in a motor vehicle in particular, the rotation uniformity of the crankshaft is not only a function of anomalies in the combustion process, but road bumps are other disturbance factors that exert an influence.
According to another method, an ion-current measurement was performed at the spark plug, and the obtained values were used for the analysis. The combustion processes can be detected directly in the cylinder with this method. The drawback, however, is that each spark plug must be provided with a corresponding measuring electrode, and a dedicated measuring circuit must be present.
It is the object of the invention to create a method that is sufficiently reliable for directly deriving the signals from the combustion processes in the cylinder, on the one hand, and on the other hand, requires no separate measuring circuit.
According to the invention, this object is accomplished for piston-type internal-combustion engines of the type mentioned at the outset in that the time difference between the opening time preset by the engine timing and the actual opening time of an exhaust valve is detected as the actual value, then compared to a preset tolerance-time difference as the nominal value; a signal is generated if this nominal value is not met. This method advantageously makes use of the fact that, in normal combustion, the pressure course of the internal cylinder pressure differs significantly from the pressure course of a combustion misfire. Because the internal cylinder pressure, as a friction value, influences the valve stroke of the electromagnetic actuator embodied as a spring-mass oscillating system for operating the gas-exhaust valve, an evaluation of the valve stroke movement starting from the xe2x80x9cexhaust openxe2x80x9d control edge of the engine timing yields a statement about whether combustion has occurred.
To open the gas-exhaust valve, the engine control unit (ECU) cuts off the current to the electrical closing magnet at a preset time, so the prestressed opening spring associated with the closing magnet moves the cylinder valve in the opening direction. In a normal combustion process, at the beginning of the exhaust stroke, a correspondingly high internal cylinder pressure builds up in the combustion chamber; the opening spring must overcome this pressure when the valve begins to open, so a corresponding delay occurs between the cutoff of the current to the closing magnet and the beginning of the opening movement of the valve.
If, however, a misfire prevents combustion, a correspondingly lower internal cylinder pressure is present at the beginning of the exhaust stroke, so the opening spring successfully opens the cylinder valve notably earlier. Because the engine control unit, which also actuates the electromagnetic actuators of the individual valves, builds up a plurality of control signals on time-based dependencies, it is thus possible, within the scope of the engine control unit, to use the detection of the time difference as the actual value in a normal combustion process, and, if this actual value is not met in the event of a misfire, to generate a corresponding signal that is detected in an xe2x80x9cerror managementxe2x80x9d within the scope of the on-board diagnosis device in the event of misfires, which occur sporadically anyway. Depending on the embodiment, when a preset number of ascertained misfires is not met within a predetermined time span, the on-board diagnosis device releases a corresponding error signal, for example through a signal light on the dashboard.
The method can be modified in a first embodiment in that at least one sensor, which is associated with a movable part of the electromagnetic actuator connected to the exhaust valve, detects the actual beginning of motion or displacement of the gas-exhaust valve during the opening stroke. With this method, the information regarding whether combustion has to take place can be read as a functioning time, directly from the path course of the opening gas-exhaust valve. If the current supply to the closing magnet of the electromagnetic actuator has been cut off, the moving part of the gas-exhaust valve is delayed. This is due to the fact that the gas-exhaust valve cannot open until a pressure level that is defined by the deflection of the opening spring and the valve size, and the internal cylinder pressure, is attained. This delay time can be detected directly as the actual value by a corresponding sensor at the electromagnetic actuator.
The path course of the exhaust valve as a function of time is further influenced by delay times occurring during the flight time. These delays result essentially from counterforces arising from the out-flowing gas during the movement acting on the valve. If the definitive delay between the cutoff of the retaining current at the closing magnet and the start of movement of the gas-exhaust valve is determined for a normal combustion process as a function of the control time and an assumed internal cylinder pressure, then the actual value derived from the delay times in a normal combustion process cannot fall below a preset limit value. If this is the case, however, a misfire has occurred. The required sensor, as a high-resolution path sensor, e.g., an eddy-current sensor, is advantageously disposed at the actuator, and detects, for example, the movement of the guide rod of the actuator armature. A path sensor of this type can be provided at an actuator for regulating the impact speed of the armature against the pole faces of the magnets. The actual value that is read off for recognizing misfires does not, however, interfere with the current regulation by the engine control unit for actuator operation, because the detection of the actual value for this diagnosis is performed over the first half of the valve movement, while the regulation of the current supply of the capturing opening magnet is effected during the second half of the valve movement, and thus also of the armature movement.
In another embodiment of the method according to the invention, it is provided that the change in the course of the voltage at the closing magnet of the actuator of the gas-exhaust valve is detected as a function of time after the retaining current has been cut off, and if a preset minimum value is exceeded, a signal is released. This method utilizes the fact that, after the retaining current is cut off at the closing magnet, the voltage sinks initially, but rises again due to electro-inductive processes when the armature releases from the pole face of the closing magnet and begins to move. At a higher cylinder pressure, as is the case during a normal combustion process, this release process takes place more slowly. The maximum of this rise is thereby shifted by a corresponding time span. The delayed release of the armature from the pole face also causes interim changes in the field conditions such that the residual voltage in the coil of the closing magnet drops again, and the voltage increase is therefore relatively small. If, however, the internal cylinder pressure is lower due to a misfire, the armature can be released from the pole face earlier and at a higher speed, so the maximum of the voltage increase is attained earlier, on the one hand, and a higher value is attained, on the other hand. Again, a corresponding signal can be derived from the ascertainable voltage difference.
The method according to the invention possesses the advantages of a highly-dynamic recognition of misfires, i.e., the misfires can be detected according to individual cycles. A further advantage of the method is that no additional sensor is required; the sensors present anyway at the actuators can be used to regulate the impact speed, or the values for the voltage course, which can be detected in the engine control unit anyway, can be used. A further advantage is that the method of the invention can be used in I-cycle methods, that is, when the engine control unit deactivates individual cylinders under predeterminable conditions. This is effected by the cutoff of the fuel supply to the cylinder, and the cutoff of the ignition, so the piston-type internal-combustion engine operates with a reduced number of cylinders. In this mode, the rotation uniformity is correspondingly high, so this factor can no longer be used to recognize misfires. The method can also be used with highly-dynamic load increases. The signal detection must be performed anyway for a valve-failure diagnosis, and/or for current regulation, so the occurring signals can be additionally evaluated within the computer of the electronic engine timing.